Media utilized for storage of information in data processing, computing, and other electronic systems represent diverse technologies and take on many forms. Familiar examples of such devices are semiconductor memories and magnetic disks and tapes. However, the media have a common characteristic, in that an increase in their storage capacity per unit of area commonly increases the occurrence of defects in the media. Defects are flaws in the media that make one or more portions of the media unsuitable for information storage. Defects are commonly the result of the presence of dust particles and other aberrations in the disk manufacturing process.
As the storage capability of bulk storage media is increased, it becomes more difficult and more expensive to manufacture flawless storage devices, and it becomes very costly to have to scrap all of the flawed devices. Consequently, schemes are needed that permit the utilization of flawed storage media.
To achieve this end, it is common to reserve a predetermined area of the storage medium to act as a spare that can substitute for defective areas of the medium. In the prior art schemes the manufacturer may substitute the spare storage areas for the defective areas fixedly and unchangeably, for example, by breaking the appropriate electrical connections in semiconductor memory chips. This type of substitution has the disadvantage that newly occurring or discovered defects in the medium cannot be compensated. As a result of these new defects, the medium may again become unutilizable.
Instead of making a fixed substitution of spare storage for defective storage, the manufacturer may merely flag defects in the medium and thereby redirect users to the area of spare storage that is substituting for the defective area. For example, in a magnetic disk or tape, a flag may be recorded at the defect, or at the beginning of a quantized portion of storage that includes the defect, which flag indicates the location of the defect and either implicitly or explicitly specifies the address of the storage which is substituting for the defective storage. A disadvantage of this approach is that it degrades the performance potential of an information storage system, because provision must be made for the significant time delay involved in twice accessing the medium to arrive at the desired storage location: firstly accessing the desired storage location, only to find it flagged as defective, and secondly accessing the spare storage location which is substituting for the defective location.